In connection with different keys, keypads and keyboards, it has been known to apply technology based on optoelectronics, the use of light guides and optical waveguides.
U.S. Pat. publication No. 4,701,747 discloses a solution based on a light emitting diode, in which the light emitting diode detects infrared light emitted from a light source and reflected from a finger. The detection corresponds to a keystroke, and such a system is constructed for each key. U.S. Pat. publication No. 4,931,794, in turn, presents a system implemented in connection with movable keys, comprising light emitting and light receiving elements. A key which has been pressed down is detected on the basis of a reflection of light caused by the key or a shading caused on the path of light.
Photoconductors have also been used for illuminating key caps of a keypad or for illuminating the background of a display. U.S. Pat. No. 6,026,283 presents a perforated photoconductor to be placed under the pad and to be illuminated by means of several light emitting diodes. The light escapes from the photoconductor and illuminates the pad from below. A circuit board underneath the photoconductor is provided with contact and switch surfaces for the keys of the pad. U.S. Pat. No. 5,963,434 discloses a corresponding structure which also comprises a reflecting surface placed on the circuit board and under the display, to intensify the reflection of light to the keypad or the display.
As portable electronic devices and particularly mobile phones become smaller, it becomes more difficult to place components in the device. Particularly the movement related to a keystroke increases the thickness of the device. Also, the function of key contact surfaces and the metal key domes functioning in connection with them is sensitive to corrosion, which impairs their reliability. The aim is thus to eliminate mechanical structures and to provide thin and integrated components.
Various membrane keys and solutions based on capacitive function are known, but they involve the problem of high power consumption, wherein they are not always suitable for electronic devices operated by batteries.